Many known modern cooking ranges feature a built-in electric gas-lighter, which is operated manually 64 means of a pushbutton to produce a spark to light the flame.
The most commonly-used ranges with built-in electric gas-lighters are of the type indicated by 1 in FIG. 1, which comprises four gas burners 2 arranged in a square and each flanked by a respective ceramic-coated electrode 3. Electrodes 3 define two pairs of output terminals of an electric gas-lighter 4 shown schematically and only as regards the output circuit. When operated, gas-lighter 4 generates a spark between each electrode 3 and the outer body (grounded together with the entire metal surface of the range) of the corresponding burner 2; and the spark lights the flame of the burner/s 2 supplied with gas.
FIG. 2 shows a complete circuit diagram of a known type of gas-lighter 4.
In addition to electrodes 3, gas-lighter 4 comprises a first and a second input terminal 7, 8 connected to a supply line (not shown); and a current-discharge generating circuit 5 interposed between input terminals 7, 8 and electrodes 3, and for producing the sparks on electrodes 3.
Circuit 5 comprises an input resistor 9 connected to terminal 7; and a rectifying diode 10 having the anode connected to resistor 9, and the cathode connected to a first intermediate node 11.
Circuit 5 also comprises a discharge capacitor 12 located between first intermediate node 11 and a second intermediate node 13 shortcircuited with second input terminal 8; a known voltage discharger 15 (e.g. a Sidac high-energy, solid-state gas tube) parallel with the branch defined by capacitor 12; and, in series with discharger 15, the primary winding 16 of a transformer 17. Transformer 17 also comprises two identical secondary windings 18, each having far more turns than primary winding 16, and the terminals of each of which have a pair of electrodes 3 of the type described above.
Gas-lighter 4 operates as follows.
When the gas-lighter 4 circuit is connected to the supply line, an initial transient state occurs in which capacitor 12 is charged to a threshold voltage value V.sub.TH equal to the ignition threshold value of discharger 15, after which, a discharge current Isc of extremely high intensity (e.g. 150-280 A) flows along a discharge path extending through primary winding 16 of transformer 17 and terminating at capacitor 12. At the terminals of primary winding 16, a discharge voltage V1 (e.g. of 400 V) is generated during the discharge transient (lasting a few microseconds) and induces, at the terminals of secondary windings 18, a discharge voltage V2 much higher than V1 (e.g. 28 kV); and, for each secondary winding 18, voltage V2 is sufficient to produce a spark between each electrode 3 and the outer body of respective burner 2, which is accompanied by instantaneous current flow between the two burners 2 of each pair of electrodes 3, and through the metal surface of cooking range 1.
Gas-lighters 4 of the above type have the drawback of generating, during the discharge transient producing the sparks, severe electromagnetic noise above the limits laid down by European standards (EN55014 and following). FIG. 3 shows the result of an electromagnetic compatibility test to determine the voltage value between input terminals 7 and 8 during the discharge transient. The voltage values, expressed in dB.mu.V, are measured in the 0.15 to 30 MHz frequency range; the regular, substantially horizontal line in the graph indicates the prescribed voltage limit, and the jagged line the measured voltage, which, as can be seen, exceeds the limit over the entire frequency range considered.
One proposed solution to the problem is to fit gas-lighter 4 with an electronic filter to reduce the electromagnetic noise during the discharge transient and so obtain a low-noise gas-lighter 4a as shown in FIG. 4. Gas-lighter 4a comprises an electronic filter 20 interposed between terminals 7, 8 and a circuit 5a equivalent to circuit 5 but having no resistor 9. Filter 20 comprises two capacitors 21a and 21b located between a node 22 connected to the anode of diode 10, and a node 23 shortcircuited with node 13. More specifically, capacitors 21a and 21b are located between respective nodes 22 and 23 and a common node 24 which is the ground. Filter 20 also comprises a pair of decoupling resistors 25 towards the mains, a first of which is located between input terminal 7 and node 22, and a second of which is located between input terminal 8 and node 23. Filter 20 defines a preferential path by which to discharge the energy produced during the transient state. More specifically, said energy is conveyed by capacitors 21a and 21b directly towards ground to reduce the electromagnetic emissions emitted by the circuit.
Though filter 20 indeed provides for reducing the noise level generated during operation to well below the prescribed limit, gas-lighter 4a fitted with filter 20 is not without further drawbacks.
First, the ground connection of capacitors 21a and 21b may result in the entry into the gas-lighter 4a circuit of electromagnetic noise generated by other electric devices and traveling along the ground lines, or of the discharge energy at electrodes 3. Second, though minimum for each gas-lighter 4a, the expense of providing a ground cable is far from negligible on a mass-production scale, as in the household appliance industry.